Scale of the Cosmos

1
Scale of the Cosmos

• Scientific Notation
• Powers of Ten
• Examples
• 300 3.0 x 102
• 3000 3.0 x 103
• 0.03 3/100 3.0 x 10-2
• Why do we need this?
• Radius of the Earth 6378 km
• Distance to the Sun 149,597,900 km
• Mass of Earth 5.976 x 1024 kg
• Mass of hydrogen atom 1.674 x 10-24 kg

2
Some new terms

• Light-Year distance light travels in one year
• 9.46 x 1015 m
• Note Distance velocity x time, so
• Light-year speed of light x one year
• Astronomical Unit average distance between the
  Earth and the Sun (1.5 x 1011 m)

3
A bit of an Overview

• We Live
• on a Planet Earth
• Which is part of a Solar System
• Including the Sun
• Which is a Star
• Part of a Galaxy
• Of which there are billions!

4
(No Transcript)
5
The Night Sky

• Constellations star patterns, Orion, Scorpius
• There are also asterisms like the Big Dipper
• Used by ancient cultures
• I.A.U. lists 88 official constellations
• The stars that make up the constellations are not
  usually physically connected.
• Within a constellation
• Brightest star is alpha (a), next beta (ß)then
  numbers (1,2,) e.g. 47 Ursa Majoris

6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
MODELS

• Scientific Models
• A mental or mathematical conception
• Useful for describing, or understanding how
  things work
• Example the Celestial Sphere
• Some terms zenith, nadir, celestial poles (North
  and South), celestial equator, horizon.
• Need some math here
• Circle 360 degrees
• 1 degree 60 minutes of arc
• 1 minute of arc 60 seconds of arc

10
(No Transcript)
11
(No Transcript)
12

• Angular distance as along the surface of a
  sphere
• Angular diameter The Moons is 0.5 degrees
• How do Earth longitude and latitude correspond to
  Celestial coordinates?
• Washington, DC 39? north latitude
• So, 39? N celestial latitude is directly
  overhead (at the zenith)
• How far is the North Celestial Pole above the
  horizon?
• How about the Celestial Equator (when facing due
  South?)

13
(No Transcript)
14

• Circumpolar does not go below horizon
• At our latitude, stars with celestial latitudes
  from 51? to 90? are circumpolar
• At the North Pole, stars from 0? to 90? are.
• What about at the Equator?
• Some other terms Celestial Latitude
  declination, Celestial longitude Right
  Ascension
• Near the North Celestial Pole the North Star
  Polaris (also known as a Ursa Minoris)

15
Precession

• The Earths axis is inclined 23.5? from vertical
  (actually relative to its direction of motion)
• Due to the Earths equatorial bulge, plus the
  pull of the Moon and the Sun (more about gravity
  later) the axis precesses
• Like a tilted spinning top
• As a result, over time, the axis will trace a
  circle on the sky and you get different North
  Stars.

16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
Brightness of Stars

• Magnitude Scale first used by Hipparchus (160
  127 B.C.)
• 1st magnitude brightest
• Then 2nd, 3rd .....5th
• 6th magnitude is the faintest you can see with
  the naked eye
• Each difference in magnitude is a factor of 2.512
  in Intensity, or
• Intensity of Star A / Intensity of Star B
  2.512(magnitude of B magnitude of A)

20

• What is Intensity ?
• Light Energy that hits one square meter in one
  second
• Sun has a magnitude of -26.5 (need negative
  numbers for objects brighter than 1st magnitude)
• Faintest stars that can be detected with the
  Hubble Space Telescope magnitude of 28
• What were using here is
• Apparent Visual Magnitude
• What do we mean by this?

21
Movement of Celestial Bodies

• Rotation of a body about an axis
• Revolution motion of a body around a point
  located outside the body
• Example
• The Earth (we now know) rotates on its axis once
  per day, while it revolves around the Sun once
  per year.

22

• During a year, the Sun moves eastward against the
  background of the stars.
• On January 1st Sun is in the constellation
  Sagittarius
• On March 1st in Aquarius
• The path of the Sun on the sky is called the
  Ecliptic
• The Ecliptic is a great circle which means it
  bisects a sphere (the Celestial Sphere) through
  its center.
• Note the Celestial Equator is another great
  circle
• Why dont the Ecliptic and the Celestial lie
  right on top of each other?

23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26

• Since the Earths axis is tilted
• Equinoxes where ecliptic crosses the equator
  (vernal and autumnal)
• Solstices when the Suns at its most northern
  point summer solstice at most southern point
  winter solstice
• declination of Sun is 00 at equinoxes
• , or 23.50 at the solstices
• Note equinox equal night
• Solstice standing sun

27
(No Transcript)
28
Seasons

• The practical consequence of the Earths tilt
• Different Seasons
• Summer more daylight, less spreading of
  Sunlight (Sun is higher in the sky)
• Which means greater Intensity
• Note Hottest days occur after the summer
  solstice Why??

29
(No Transcript)
30

• Earth is closest to the Sun on Jan. 4
  perihelion
• We are furthest from the Sun on July 4 aphelion
• Why doesnt this have a bigger effect?
• Difference is only a couple of
• Other planets have more eccentric orbits, hence
  greater ranges in the intensity of the Sunlight

31
Climate

• Climate has changed over time,
• For example there have been ice ages currently
  the Earth appears to be warming
• Milankovitch hypothesis the climate is affected
  by orbit, precession, and inclination
• Note the inclination varies from 220 to 240 over
  a period of 41,000 years

32

• Proof? Ocean temperatures, based on seafloor
  samples show correlation with orbital and
  rotational variations
• But. Devils Hole, Nevada a deep, water-filled
  crack, showed evidence that the end of the last
  ice was 1000s of years too early
• Explanation Devils Hole shows evidence of local
  climate change seafloor shows effects of global
  change